1. Field of the Invention
The present invention relates to dielectric ceramics, methods for making and evaluating the same, and monolithic ceramic electronic components. In particular, the present invention relates to thin monolithic ceramic electronic components such as thin monolithic ceramic capacitors.
2. Description of the Related Art
Monolithic ceramic capacitors, as an example of monolithic ceramic electronic components relating to the present invention, are typically produced as follows.
Green ceramic sheets, each composed of a dielectric ceramic material and provided with an internal electrode pattern of a conductive material, are prepared. The dielectric ceramic material may comprise BaTiO3, for example.
A plurality of green ceramic sheets, including the above sheets provided with the internal electrode patterns, is stacked and is thermally compressed to form a green composite.
The green composite is fired to prepare a sintered composite, which has internal electrodes formed of the above-described conductive material.
External electrodes are formed on outer faces of the composite so that the external electrodes are electrically connected to predetermined internal electrodes. The external electrodes are formed, for example, by applying a conductive paste containing a conductive metal powder and a glass frit on the outer faces of the composite and baking the composite. A monolithic capacitor is thereby formed.
In order to reduce production costs of the monolithic ceramic capacitors, relatively inexpensive base metals such as nickel and copper are often used nowadays as the conductive materials for the internal electrodes. Unfortunately, the green composite must be fired in a neutral or reducing atmosphere to prevent oxidation of the base metal in the production of monolithic ceramic capacitors having such internal electrodes formed of base metals. As a result, the dielectric ceramic used in the monolithic ceramic capacitor must have resistance to reducing atmosphere.
BaTiO3-rare earth oxide-Co2O3 compositions for such dielectric ceramics having resistance to reducing atmosphere are disclosed in Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067, and 5-9068. Dielectric ceramics having a high dielectric constant, a small change in dielectric constant with temperature and a long life at high-temperature load are disclosed in Japanese Unexamined Patent Application Publication Nos. 6-5460 and 9-270366.
Trends toward miniaturization and higher capacitance are noticeable in monolithic ceramic capacitors with the rapid miniaturization of electronic components as a result of recent great advances in electronics technologies.
The requirements regarding reliability for dielectric ceramics which are fired in an atmosphere which does not oxidize base metals used in internal electrodes are a high dielectric constant, a small change in dielectric constant with temperature and time, and high electrical insulation for thinner dielectric ceramic layers. The above-described known dielectric ceramics, however, do not completely satisfy these requirements.
For example, the dielectric ceramics disclosed in Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067, and 5-9068 above satisfy the X7R characteristics in the EIA Standard and exhibit high electrical insulation, but do not always satisfy the demands of the market, namely, they may be sufficiently reliable, when the thicknesses of the dielectric ceramics are reduced to about 5 xcexcm or less and particularly 3 xcexcm or less.
In the dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 6-5460, the particle size of the BaTiO3 powder used is large. Thus, its reliability decreases and the change in electrostatic capacitance with time increases as the thickness of the dielectric ceramic layer decreases.
Also, the reliability of the dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 9-270366 decreases and the change in electrostatic capacitance with time increases while applying a DC voltage as the thickness of the dielectric ceramic layer decreases.
When the same rated voltage is applied to a dielectric ceramic layer having a reduced thickness, which agrees with miniaturization and higher capacitance requirements of the monolithic ceramic capacitor, a larger electric field is applied to each layer of the dielectric ceramic. Thus, the insulating resistance at room or high temperature decreases, resulting in significantly decreased reliability. Accordingly, the rated voltage must be reduced when the thickness of the dielectric ceramic layers in the known dielectric ceramic is reduced.
There have been demands that monolithic ceramic capacitors have high insulation resistance in high electric fields and have high reliability, and that they can be used at high rated voltages even when the thicknesses of the dielectric ceramic layers are reduced.
It is known that the electrostatic capacitance of a monolithic ceramic capacitor varies over time because a DC voltage is applied in use. As the thickness of the dielectric ceramic layers decreases, the DC electric field per dielectric ceramic layer increases. As a result, the electrostatic capacitance changes more significantly over time.
Thus, there have been demands that monolithic ceramic capacitors have a small change in electrostatic capacitance when a DC voltage is applied in use.
Also, monolithic ceramic electronic components other than the monolithic ceramic capacitors have the above-described problems and demands.
Accordingly, an object of the present invention is to provide a dielectric ceramic exhibiting a high dielectric constant, small changes in dielectric constant with temperature and over time when a DC voltage is applied in use, a high product of insulation resistance and electrostatic capacitance (CR product), and a prolonged lifetime, in terms of insulation resistance, under accelerated testing at high temperature and high voltage.
Another object of the present invention is to provide a method for making the dielectric ceramic.
Another object of the present invention is to provide a method for evaluating the dielectric ceramic in which dielectric ceramics having the above superior characteristics can be readily and efficiently selected, for example, in a designing process.
Another object of the present invention is to provide a monolithic electric component comprising the above dielectric ceramic.
The present invention is directed to a dielectric ceramic having a ceramic structure comprising crystal grains and grain boundaries between the crystal grains, the crystal grains comprising a main component represented by the formula ABO3 and an additive containing a rare earth element wherein A is at least one of barium, calcium and strontium, barium being an essential element, and B is at least one of titanium, zirconium and hafnium, titanium being an essential element.
This dielectric ceramic further satisfies the following conditions: (1) the average rare earth element concentration in the interior of the crystal grains is about 0.5 or less the average rare earth element concentration at the grain boundaries, and (2) about 20% to 70% of the crystal grains have a rare earth element concentration in the center of the crystal grain of at least about {fraction (1/50)} the maximum rare earth element concentration in a region extending inward from the surface by a distance corresponding to about 5% of the diameter of the crystal grain.
This dielectric ceramic exhibits a high dielectric constant, small changes in dielectric constant with temperature and over time when a DC voltage is applied in use, a high product of insulation resistance and electrostatic capacitance (CR product), and a prolonged lifetime, in terms of insulation resistance, under accelerated testing at high temperature and high voltage.
A thin monolithic ceramic electronic component including dielectric ceramic layers composed of this dielectric ceramic is highly reliable for a long time.
The present invention is also directed to a method for making such a dielectric ceramic. The method for making the dielectric ceramic comprises the steps of mixing fractions of AO, BO2 and the rare earth element, calcining the mixture in air, and pulverizing the mixture to prepare a modified ABO3 powder in which the rare earth element is present in the interiors of the particles; mixing the remaining fractions of the AO and BO2, calcining the mixture in air, and pulverizing the mixture to prepare an ABO3 powder in which the rare earth element is not present in the interiors of the particles; and mixing the modified ABO3 powder, the ABO3 powder and the remaining fraction of the rare earth element, and firing the mixture.
Since the ABO3 powder, the modified ABO3 powder and the rare earth element are mixed at two stages, the above-described concentration profile of the rare earth element is readily achieved in the crystal grains. In conventional one-shot mixing, such a concentration profile is barely achieved.
Furthermore, the present invention is directed to a method for evaluating a dielectric ceramic that has a ceramic structure comprising crystal grains and grain boundaries between the crystal grains, the crystal grains comprising a main component represented by the formula ABO3 and an additive containing a rare earth element wherein A is at least one of barium, calcium and strontium, barium being an essential element, and B is at least one of titanium, zirconium and hafnium, titanium being an essential element.
This method comprises the steps of measuring the average rare earth element concentration in the interiors of the crystal grains and the average rare earth element concentration at the grain boundaries; determining whether or not a first condition that the average rare earth element concentration in the interior of the crystal grains is about xc2xd or less the average rare earth element concentration at the grain boundaries is satisfied; measuring the rare earth element concentration in the center of each crystal grain and the maximum rare earth element concentration in a region extending inward from the surface by a distance corresponding to about 5% of the diameter of the crystal grain; determining whether or not a second condition that about 20% to 70% of the crystal grains each have a rare earth element concentration in the center of the crystal grain which is at least about {fraction (1/50)} of the maximum rare earth element concentration in the region is satisfied; and assuming the dielectric ceramic to be nondefective when the dielectric ceramic satisfies the first and second conditions.
This method condenses a cycle of designing, making and evaluation of a dielectric ceramic.
The present invention is also directed to a monolithic ceramic electronic component comprising a composite comprising a plurality of stacked dielectric ceramic layers; and internal electrodes formed along predetermined interfaces between the dielectric ceramic layers, the dielectric ceramic layers comprising the above-described dielectric ceramic.
Preferably, the internal electrodes comprise a base metal.
Since the dielectric ceramic according to the present invention exhibits high resistance to reducing environments, the base metal can be used as a conductive component of the internal electrodes.
The monolithic ceramic electronic component is preferably a monolithic ceramic capacitor. In such a case, the monolithic ceramic electronic component further comprises a first external electrode and a second external electrode formed on outer faces of the composite, wherein the internal electrodes are arranged in the stacking direction of the composite and are alternately and electrically connected to the first external electrode and the second external electrode to define the monolithic ceramic capacitor.
This monolithic ceramic capacitor has a large capacitance regardless of its compactness and can be used at conventional rated voltages. Thus, the thickness of the dielectric ceramic layers in the monolithic ceramic capacitor can be reduced to, for example, about 1 xcexcm without problems.